In general, carbon fiber is dispersed in a matrix such as resin, to thereby impart electrical conductivity and thermal conductivity thereto. Vapor grown carbon fiber (hereinafter may be abbreviated as “VGCF”) is very useful, since, even when a small amount of the carbon fiber is added to a resin, the resultant resin composition exhibits greatly enhanced electrical conductivity and thermal conductivity, and therefore workability of the resin composition is not lowered, and the surface appearance of a molded product formed from the composition is not impaired (U.S. Pat. No. 5,643,990). As has been known, when carbon fiber having a large number of branches is added to a material, the electrical conductivity of the material is enhanced (WO 02/049412). Therefore, demand has arisen for production of carbon fiber having a large number of branches.
As one method for producing carbon fiber through a vapor phase process, a gasification method has been proposed (U.S. Pat. No. 4,572,813). In the gasification method, a solution of an organic substance in which an organo-transition metallic compound is dissolved is gasified, to thereby allow reaction to proceed at high temperature within a heating zone. This gasification method produces carbon fiber having a small number of branches. Meanwhile, there has been proposed a method for producing branched carbon fiber by spraying droplets of a raw material onto the wall of a reaction tube (Japanese Patent No. 2778434). In this method, droplets of a raw material are fed to the reaction tube wall, to thereby grow carbon fiber on the reaction tube wall. After the reaction tube wall is covered with the thus-grown carbon fiber, droplets of the raw material are sprayed onto the carbon fiber, a catalyst is generated on the carbon fiber, and, on the carbon fiber serving as a substrate, fresh carbon fiber is grown to thereby form branches, whereby branched carbon fiber is produced at high yield.